:mod:`xdrlib` --- Encode and decode XDR data
============================================
.. module:: xdrlib
:synopsis: Encoders and decoders for the External Data Representation (XDR).
.. index::
single: XDR
single: External Data Representation
**Source code:** :source:`Lib/xdrlib.py`
--------------
The :mod:`xdrlib` module supports the External Data Representation Standard as
described in :rfc:`1014`, written by Sun Microsystems, Inc. June 1987. It
supports most of the data types described in the RFC.
The :mod:`xdrlib` module defines two classes, one for packing variables into XDR
representation, and another for unpacking from XDR representation. There are
also two exception classes.
.. class:: Packer()
:class:`Packer` is the class for packing data into XDR representation. The
:class:`Packer` class is instantiated with no arguments.
.. class:: Unpacker(data)
``Unpacker`` is the complementary class which unpacks XDR data values from a
string buffer. The input buffer is given as *data*.
.. seealso::
:rfc:`1014` - XDR: External Data Representation Standard
This RFC defined the encoding of data which was XDR at the time this module was
originally written. It has apparently been obsoleted by :rfc:`1832`.
:rfc:`1832` - XDR: External Data Representation Standard
Newer RFC that provides a revised definition of XDR.
.. _xdr-packer-objects:
Packer Objects
--------------
:class:`Packer` instances have the following methods:
.. method:: Packer.get_buffer()
Returns the current pack buffer as a string.
.. method:: Packer.reset()
Resets the pack buffer to the empty string.
In general, you can pack any of the most common XDR data types by calling the
appropriate ``pack_type()`` method. Each method takes a single argument, the
value to pack. The following simple data type packing methods are supported:
:meth:`pack_uint`, :meth:`pack_int`, :meth:`pack_enum`, :meth:`pack_bool`,
:meth:`pack_uhyper`, and :meth:`pack_hyper`.
.. method:: Packer.pack_float(value)
Packs the single-precision floating point number *value*.
.. method:: Packer.pack_double(value)
Packs the double-precision floating point number *value*.
The following methods support packing strings, bytes, and opaque data:
.. method:: Packer.pack_fstring(n, s)
Packs a fixed length string, *s*. *n* is the length of the string but it is
*not* packed into the data buffer. The string is padded with null bytes if
necessary to guaranteed 4 byte alignment.
.. method:: Packer.pack_fopaque(n, data)
Packs a fixed length opaque data stream, similarly to :meth:`pack_fstring`.
.. method:: Packer.pack_string(s)
Packs a variable length string, *s*. The length of the string is first packed
as an unsigned integer, then the string data is packed with
:meth:`pack_fstring`.
.. method:: Packer.pack_opaque(data)
Packs a variable length opaque data string, similarly to :meth:`pack_string`.
.. method:: Packer.pack_bytes(bytes)
Packs a variable length byte stream, similarly to :meth:`pack_string`.
The following methods support packing arrays and lists:
.. method:: Packer.pack_list(list, pack_item)
Packs a *list* of homogeneous items. This method is useful for lists with an
indeterminate size; i.e. the size is not available until the entire list has
been walked. For each item in the list, an unsigned integer ``1`` is packed
first, followed by the data value from the list. *pack_item* is the function
that is called to pack the individual item. At the end of the list, an unsigned
integer ``0`` is packed.
For example, to pack a list of integers, the code might appear like this::
import xdrlib
p = xdrlib.Packer()
p.pack_list([1, 2, 3], p.pack_int)
.. method:: Packer.pack_farray(n, array, pack_item)
Packs a fixed length list (*array*) of homogeneous items. *n* is the length of
the list; it is *not* packed into the buffer, but a :exc:`ValueError` exception
is raised if ``len(array)`` is not equal to *n*. As above, *pack_item* is the
function used to pack each element.
.. method:: Packer.pack_array(list, pack_item)
Packs a variable length *list* of homogeneous items. First, the length of the
list is packed as an unsigned integer, then each element is packed as in
:meth:`pack_farray` above.
.. _xdr-unpacker-objects:
Unpacker Objects
----------------
The :class:`Unpacker` class offers the following methods:
.. method:: Unpacker.reset(data)
Resets the string buffer with the given *data*.
.. method:: Unpacker.get_position()
Returns the current unpack position in the data buffer.
.. method:: Unpacker.set_position(position)
Sets the data buffer unpack position to *position*. You should be careful about
using :meth:`get_position` and :meth:`set_position`.
.. method:: Unpacker.get_buffer()
Returns the current unpack data buffer as a string.
.. method:: Unpacker.done()
Indicates unpack completion. Raises an :exc:`Error` exception if all of the
data has not been unpacked.
In addition, every data type that can be packed with a :class:`Packer`, can be
unpacked with an :class:`Unpacker`. Unpacking methods are of the form
``unpack_type()``, and take no arguments. They return the unpacked object.
.. method:: Unpacker.unpack_float()
Unpacks a single-precision floating point number.
.. method:: Unpacker.unpack_double()
Unpacks a double-precision floating point number, similarly to
:meth:`unpack_float`.
In addition, the following methods unpack strings, bytes, and opaque data:
.. method:: Unpacker.unpack_fstring(n)
Unpacks and returns a fixed length string. *n* is the number of characters
expected. Padding with null bytes to guaranteed 4 byte alignment is assumed.
.. method:: Unpacker.unpack_fopaque(n)
Unpacks and returns a fixed length opaque data stream, similarly to
:meth:`unpack_fstring`.
.. method:: Unpacker.unpack_string()
Unpacks and returns a variable length string. The length of the string is first
unpacked as an unsigned integer, then the string data is unpacked with
:meth:`unpack_fstring`.
.. method:: Unpacker.unpack_opaque()
Unpacks and returns a variable length opaque data string, similarly to
:meth:`unpack_string`.
.. method:: Unpacker.unpack_bytes()
Unpacks and returns a variable length byte stream, similarly to
:meth:`unpack_string`.
The following methods support unpacking arrays and lists:
.. method:: Unpacker.unpack_list(unpack_item)
Unpacks and returns a list of homogeneous items. The list is unpacked one
element at a time by first unpacking an unsigned integer flag. If the flag is
``1``, then the item is unpacked and appended to the list. A flag of ``0``
indicates the end of the list. *unpack_item* is the function that is called to
unpack the items.
.. method:: Unpacker.unpack_farray(n, unpack_item)
Unpacks and returns (as a list) a fixed length array of homogeneous items. *n*
is number of list elements to expect in the buffer. As above, *unpack_item* is
the function used to unpack each element.
.. method:: Unpacker.unpack_array(unpack_item)
Unpacks and returns a variable length *list* of homogeneous items. First, the
length of the list is unpacked as an unsigned integer, then each element is
unpacked as in :meth:`unpack_farray` above.
.. _xdr-exceptions:
Exceptions
----------
Exceptions in this module are coded as class instances:
.. exception:: Error
The base exception class. :exc:`Error` has a single public attribute
:attr:`msg` containing the description of the error.
.. exception:: ConversionError
Class derived from :exc:`Error`. Contains no additional instance variables.
Here is an example of how you would catch one of these exceptions::
import xdrlib
p = xdrlib.Packer()
try:
p.pack_double(8.01)
except xdrlib.ConversionError as instance:
print 'packing the double failed:', instance.msg